As one way of inspecting non-destructively internal defects of mechanical components and the like, there exists the method of transmitting X-rays into the interior of the target object to acquire images thereof (method of inspection by X-ray radiography). In particular, X-ray computed tomography systems exemplified by industrial X-ray computed tomography (CT) are capable of highly accurate tests because they can output detailed images of the interior of the target object unlike radiographic images from radiographic tests.
Also, there has been a growing need for the non-destructive inspection by X-ray computed tomography of the piping installed in power plants such as nuclear and thermal power plants, chemical plants, and oil plants. The piping of these plants is often installed in narrow locations. It is difficult to apply industrial-use X-ray computed tomography systems to the plant piping because the systems require rotating operations and have a large equipment scale. Japanese Unexamined Patent Publication No. 2008-275352 (Patent Literature 1) discloses an X-ray computed tomography method that can be applied to the plant piping. Conventional X-ray computed tomography systems perform their image reconstruction computing process based on a plurality of projection data acquired by imaging the object to be tested (called the test object hereunder) at angles of 180 degrees plus a radiation angle or at angles of 360 degrees in general, whereby tomographic or stereoscopic images are constructed. On the other hand, what is disclosed in Patent Literature 1 is a tomography method called laminography. The disclosed method involves using as a priori information such known information as shape information about the test object from among a plurality of projection data acquired at smaller angles than the angles necessary for X-ray computed tomography systems so as to perform an image reconstruction computing process supporting smaller angles, whereby tomographic or stereoscopic images can be constructed. In piping inspections that use laminography, images are acquired by having a radiation source and a radiation detector moved in parallel to the longitudinal direction of the piping.
Object examinations that use X-ray computed tomography are often targeted to test objects made up of a plurality of materials each. For example, whereas the piping inspection of a plant requires verifying the state of the piping interior, the pipes are wrapped with heat insulating materials which in turn are covered with outer castings. In piping inspections involving X-ray computed tomography, it is desired to obtain images of the piping interior for evaluation without removing the heat insulating material and outer casting. Generally, X-ray computed tomography requires positioning the entire test object within a field of vision for imaging. In image reconstruction computations for constructing tomographic images, not only the test object but also other regions need to be calculated. This requires an extensive calculating area that will increase the amount and the time of computations in addition to a growing storage area to be needed.
In view of the problems above, there have been conceived methods of using some appropriate means to estimate information about the regions other than the test object (physical properties and thicknesses of materials, etc.) and to eliminate such information. Japanese Unexamined Patent Publication No. 2011-24773 (Patent Literature 2) proposes a method of estimating the information about the regions other than the test object.
The method proposed by Patent Literature 2 involves emitting X-rays to the test object using a plurality of X-ray energy bands and, based on the measurements taken of the transmitted X-rays, obtaining the ratio of the components making up the test object. However, although this method permits acquisition of the component ratio, the thicknesses of the materials involved are not obtained. Thus Patent Literature 2 discloses little about the method of tomography targeted solely to the test object among subjects to be inspected.
It is therefore an object of the present invention to provide a tomography method and a tomography system capable of tomographic imaging targeted uniquely to the test object among subjects to be inspected.